1. Field of the Invention
The present invention relates to a negative photosensitive material containing a photosensitive polyimide precursor (polyamic acid) for formation of a negative pattern, and a photosensitive board employing the negative photosensitive material. The present invention further relates to a negative pattern forming method to be used for producing a hard disk drive suspension board and other circuit-containing suspension board. With a low linear expansion coefficient and a low hygroscopic expansion coefficient, the board produced by this method is substantially free from warpage due to hygroscopic influence. Further, the board can be subjected to polyimide etching (PI etching).
2. Description of the Related Art
In recent years, hard disk drives (hereinafter sometimes referred to as “HDD”) to be incorporated in personal computers are required to have a higher capacity and a higher information transmission speed. Such an HDD includes a so-called magnetic head, and a so-called magnetic head suspension which supports the magnetic head.
With a recent rapidly increasing demand for the higher capacity, the HDD is required to access a minute region for reading and writing, so that a distance between the magnetic head and a disk tends to be reduced. For precise control of the distance between the magnetic head and the disk, a polyimide photosensitive material having a low linear expansion coefficient and a low hygroscopic expansion coefficient is used instead of a conventional epoxy resin photosensitive material as an insulative resin for preparation of a base on which wirings are formed. On the other hand, HDDs to be mounted in smaller-size devices such as portable devices are also increasingly required to meet various demands. A disk of such an HDD for recording information has a smaller diameter and a correspondingly higher recording density. In order to access a track of the smaller-diameter disk for the data reading and the data writing, the HDD is required to slowly rotate the disk. Therefore, the relative speed (circumferential speed) of the disk with respect to a magnetic head is low, so that a suspension board should be moved toward the disk with a smaller force. Hence, the suspension board is required to have a lower rigidity.
The suspension board of the HDD typically includes a metal substrate, an insulative layer, an interconnection layer and a cover layer, which are patterned and stacked in this order. A conceivable method for imparting the suspension board with a lower rigidity is to reduce the residual proportion of the metal substrate which has a relatively high rigidity. With the reduction of the residual proportion of the highly rigid metal substrate, however, the suspension board is liable to warp. To cope with this problem, JP-A-2008-310946 proposes that a polyimide precursor having a lower hygroscopic expansion coefficient is used as a material for the insulative layer and the cover layer to suppress the warpage of the suspension board. Further, Japanese Patent No. 3332278 proposes that a wiring circuit board of a multilayer structure including a plurality of thin films and having a lower linear expansion coefficient is produced by using a polyimide precursor that is unlikely to suffer from accumulation of interlayer residual stress.
However, a material containing the polyimide precursor is inferior in gradational patternability and PI etchability (i.e., it is difficult to properly pattern the material through a gradational exposure and development process and a PI etching process) and, therefore, unsatisfactory in this regard.
In view of the foregoing, it is an object of the present invention to provide a negative photosensitive material having a low linear expansion coefficient and a low hygroscopic expansion coefficient, a photosensitive board employing the negative photosensitive material, and a negative pattern forming method which ensures excellent gradational patternability and excellent PI etchability in patterning the negative photosensitive material.